Process for hydrothermal production of sodium silicate solutions and precipitated silicas

ABSTRACT

A method is provided for treating silica sand scrubs (SSS) generated and accumulated as waste in the chloride manufacturing process of titanium dioxide pigment. A hydrothermal process is used to produce sodium silicate solutions of modulus 3.0 to 3.8, and precipitated silicas. In some embodiments, the process uses two specific principal reaction stages. A sodium silicate solution having a low SiO 2 :Na 2 O molar ratio, in the range from 2.0 to 2.8, is first produced by reaction of the SSS, as a cost-effective SiO 2  source, with aqueous caustic soda. The conversion of this intermediate sodium silicate solution of low modulus to a high SiO 2 :Na 2 O molar ratio is made possible by using a SiO 2  source that is prepared as precipitated amorphous silica from the intermediate sodium silicate solution produced above.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/880,334 filed Jul. 20, 2007, which is incorporated herein in itsentirety by reference.

FIELD

The present teachings relate to processes of producing sodium silicatesolutions and precipitated silicas.

BACKGROUND

In the chloride process for making titanium dioxide, titaniumtetrachloride is oxidized in the vapour phase to titanium dioxide. Thetitanium dioxide and other reaction products are then carried forwardthrough externally cooled conduits capable of heat exchange, where theyare cooled and coalesced. During the cooling process, the oxide can beaccreted upon the walls of the reactor and on other surfaces in thereaction zone. Accretion of TiO₂ product usually results in a number ofdeleterious effects, such as loss of product quality due to excessiveretention of the product in a high temperature environment, drop-off ofwall accretion into the main product stream, localized overheating ofequipment due to poor heat transfer through the accretion and pluggingof gas entries.

These problems resulting from the formation of TiO₂ particulate depositson the internal walls of conduits can be reduced by introducingrelatively hard granular abrasion particles into the hot suspension.Silica sand is preferably used, as granular scrubs for removing TiO₂deposits from the internal surfaces of cooling conduits containing a hotsuspension of TiO₂ particles, for example, suspension containing from0.1% to 0.5% by weight of titanium dioxide pigment. The specificparticles for such scrubbing can have average diameters in the range ofsize distribution of from about 1 mm to about 0.5 mm.

For simplicity of description, the above-described scouring particlesare herein under referred to as “scrubbing solids” or simply “scrubs.”

The scrubs are introduced before the reaction stream exits the reactorinto cooling ducts to scrub and remove build-up from the interior of theflue pipes downstream from the TiO₂ oxidation section. The reactionstream with the scrub solids is cooled.

The pigment and scrub solids are discharged from the cooler to a cycloneand then a bag filter to separate the gases. The solids are thenslurried in water.

Since such scrub particles constitute undesired grits in the pigmentproduct, it is necessary to remove them.

The slurry is screened for subsequent treatment and silica sand scrubsare separated from the titanium dioxide slurry as they can contaminatethe final titanium dioxide pigment product.

As a matter of fact, the silica sand scrubs reclaimed from a titaniumdioxide slurry resulting from a chloride manufacturing process presentsenvironmental problems as there is no means for complete reprocessingand recycling of the scrub material. Previously, no significanttechnically and economically practical use of this waste has beenprovided so the waste has either been collected and used as landfill ormoved to a dump. As the sand material becomes available in largequantities as a waste material at site, it becomes increasingly moreenvironmentally invasive and its disposal becomes more and moredifficult and expensive.

It is well recognized in this industry that any use to which thesesilica sand scrub wastes can be put would be highly beneficial.Therefore, there exists a need for a process for converting the silicondioxide content of this waste into products such as silicates and silicagel, or precipitated silicas, which represent high grade value-addedproducts exhibiting a wide range of useful applications.

There are a number of process techniques for the production of watersoluble alkali silicates. These include, inter alia, either the dryprocess or the wet process. In the dry process, quartz sand is used assource of silicon dioxide and is caused to react with soda or sodiumcarbonate in melt temperatures in the range from 1400° C. to 1500° C.and subsequently dissolved in water under pressure at elevatedtemperature in another processing step.

In the wet process, quartz-sand is made to react in a pressure reactorat a temperature of 180° C. to 250° C., under saturated steam pressurecorresponding to the temperature used, with an aqueous alkali-hydroxidesolution.

Thus, the source of silica in either of these two above processes isquartz sand. In the case of wet process, the conversion reaction issluggish and does not take place quantitatively and results in lowmodulus sodium silicate. If sodium silicate with a high SiO₂:Na₂O molarratio is to be produced, the essential prerequisite is the selectiveutilization of higher soluble silica modifications like amorphoussilicon dioxide, such as that from industrial flue dusts, from naturallyoccurring amorphous silicon dioxide containing materials andcristobalite modification of the silica sand, either available fromnature or prepared by tempering process, as source of silica in theprocess. Alongside these, industrial by-and-waste products play asalternative raw materials for the production of cheap sodium silicatesolutions.

As described above, the dry process has the disadvantages that theprocess is highly expensive both in terms of investment and maintenancecosts and in terms of energy consumption and, further, it causesconsiderable air pollution. It is characterized, in addition, by aparticularly careful selection of the silicon dioxide material,especially with a view toward the content of iron and aluminum oxides.

The following prior art processes involve conventional processingtechniques using different sources of silicon dioxide for the productionof sodium silicate solutions.

U.S. Pat. No. 4,190,632 teaches a process for producing sodium silicatesolution by treating air-borne dust containing silicon dioxide with analkali to form an alkali metal silicate solution at a temperature ofabout 60° C. to about 110° C., wherein the air-borne dust is a wasteproduct that has been removed from the flue gases originating fromsilicon metal or ferro silicon alloy production processes with particlesize below 90 micron, followed by purification by adding activatedcharcoal and/or oxidation agents such as sodium peroxide or hydrogenperoxide and filtration. The purifying agents are added 1 hour beforethe end of boiling. The flue dusts used as starting materials has highamorphous silica, SiO₂, content of 89.5% to 97.5% by weight, theremaining consisting of impurities. The sodium silicate solutionobtained had a SiO₂:Na₂O molar ratio of 4.1:1. The process ischaracterized in that very fine amorphous siliceous particles in theform of flue dust are used as source of silicon dioxide and that theproduct is subjected to purification steps involving activated charcoaland oxidizing agents.

U.S. Pat. No. 5,000,933 discloses a process for direct hydrothermalproduction of high purity sodium silicate solutions having a highSiO₂:Na₂O molar ratio by reaction of a silicon dioxide source withaqueous sodium hydroxide solutions, or with aqueous sodium silicatesolutions having a lower SiO₂:Na₂O molar ratio, characterized in thatthe silicon dioxide source provided contains a sufficient fraction ofnatural cristobalite phase, or synthetic cristobalite produced byconditioning other crystalline forms of silicon dioxide by heating at atemperature from about 1200° C. to about 1700° C. in the presence of acatalyst but below the melting point of silica, before the hydrothermaltreatment. Even with use of readily soluble natural or syntheticcristobalite containing silicon dioxide, the hydrothermal reaction hasto be carried out in a closed pressure reactor at a temperature in therange of from 200° C. to 250° C., for example, 225° C., under saturatedsteam pressures corresponding to the temperature, in order to get aSiO₂:Na₂O molar ratio of from 3.3 to 3.5:1.

It can be seen that the prior art processes described above followmethods for making sodium silicate solutions characterized in that theraw material comes as specialized product such as natural or syntheticcristobalites and in that very fine by-product and waste-product silicondioxide sources are utilized. It is specific in that the cristobalitemodification of the silica sand is used either exclusively in a singlestage reaction process or by preparing a cristobalite modification fromnatural sand through a tempering process followed by a two-stagereaction process. This means a high process cost due to the high priceof the raw material containing cristobalite. The tempering processoption involves both a highest investment and a high process risk. Asingle stage reaction process using cristobalite might exhibit thelowest investment effort but exhibits the highest operational costs dueto the need for cristobalite as a raw material.

There exists a need for a reprocessing inexpensive silica sand scrubwastes emanating from titanium dioxide production processes usingchloride technology, as a low cost starting material. There is a need toconvert this waste into high modulus sodium silicates and silicas, whichrepresent value-added products exhibiting a wide range of applications,especially for captive use for silica coating of titanium dioxidepigment and as fillers and extenders in diverse industrial products.

SUMMARY

According to various embodiments, the present teachings provide a methodfor manufacturing cost-effective quality sodium silicate solutions ofhigh modulus which involves replacing conventional silica sources withinexpensive and readily processable precursors of silica.

In accordance with the present teachings, it has been found that thevery silica sand scrubs recovered from and collected which are high insilicon content can be disposed of by converting them into silicates andsilicas as described herein.

In accordance with a feature of the present teachings, these recoveredmaterials have desirable properties as a result of their prior thermalhistory of going through a thermal treatment during their initial use asscrubbers in the oxidizer which can ease autoclave reaction in asubsequent sodium silicate production.

The main impurity in the scrubs is TiO₂ originating from TiO₂ pigmentadhering to the surface. In accordance with an embodiment of the presentteachings, the surface purification involves the TiO₂ impurity andexfoliation and dispersion thereof from the surface of the scrubs thatcan be easily achieved by subjecting to shear washing.

It has been found that surprising quality improvement can besufficiently achieved for subsequent processing when the washed anddried sand prepared as above is sieved on a 1 mm sieve. By this sievingstep only approximately 2% of the total weight of the washed sand wasremoved. Therefore, the choice of sieving under the present teachingsoffers an additional simple method of purifying the scrubs of their TiO₂impurities.

In some embodiments silica sand waste emanating from titanium dioxideproduction by the chloride route, is used for the hydrothermalproduction of water soluble sodium silicate solutions having a widerange of SiO₂:Na₂O ratios, and preferably having an Na₂O content highenough to provide a clear water solution for use.

The present teachings relate to a process for the hydrothermalproduction of sodium silicate solutions having a high SiO₂:Na₂O molarratio, wherein silica sand scrubs that have been recovered and collectedas waste from a titanium dioxide manufacturing process using chloridetechnology are used as a precursor of silica. The present teachings,also relate to a method of using silica sand scrubs that have beenreclaimed by recovery systems from a titanium dioxide manufacturingprocess involving chloride technology, and previously considered waste,as a value-added product and using it in the hydrothermal production ofhigh modulus sodium silicate solutions, and in a myriad of otherapplications. The present teachings also relate to a method ofmanagement and disposal of such waste to bring it in line with strategicwaste management systems thereby assuring environmental protection. Inaddition, the present teachings relate to a method of producingprecipitated silicas from aqueous-based sodium silicate solutionsresulting from processing silica sand scrubs reclaimed from a titaniumdioxide manufacturing process.

The present teachings overcome the aforementioned shortcomings of theprior art, and fulfills needs in the art by providing a process for thehydrothermal production of sodium silicate solutions (SS), and suchcompositions themselves, wherein silica sand scrubs that have beengenerated and collected as waste during a chloride process ofmanufacturing titanium dioxide pigment, are used as a cost-effectiveprecursor of silicon dioxide. According to various embodiments, theprocess can comprise two specific principal stages of a reactionprocess, in which sodium silicate solutions of low modulus are initiallyproduced, designated as intermediate (stage 1) sodium silicatesolutions, that can be further processed to yield a sodium silicatesolution of high molar ratio, designated as “boosted” sodium silicate(stage 2).

The products obtained from stage 1 and stage 2, can have the followingtypical characteristics, that are seen in Table-1 below:

TABLE 1 Typical Characteristics of Sodium Silicate of the presentteachings Low Modulus High Modulus Boosted Characteristics Intermediate(Stage 1) (Stage 2) °Be 47.35 40.55 Density 1.45-1.48 1.3882 SiO₂, %30.27 29.17 Na₂O, % 12.18 8.55 Dry product, % wt 42.45 37.72 SiO₂ % dryproduct 71.31 77.33 Na₂O % dry product 28.69 22.67 SiO₂:Na₂O, R 2.563.52

According to various embodiments of the present teachings, a process ofpreparing high modulus sodium silicate by a hydrothermal reaction ofwaste silica sand scrubs and caustic soda can comprise the chronologicalsteps of:

-   -   1. attacking silica sand scrubs with caustic soda to obtain        sodium silicate (SS) having a Modulus® of from 2.0 to 2.8, as an        intermediate product;    -   2. producing precipitated silica by reacting the SS from step 1        with an acidifying agent;    -   3. raising R from the range of 2.0 to 2.8 to the range of 3.0 to        3.8 by reacting SS from step 1 with precipitated silica obtained        from step 2; and    -   4. drying excess wet silica of step 2, to obtain a moisture        content of from about 5% by weight to about 50% by weight.

According to various embodiments, the process can further comprise thesteps of:

-   -   1. shear washing of waste silica sand scrubs plus filtration;    -   2. optionally mill of sand;    -   3. Filtration of residuals after hydrothermal fusion of sand and        caustic soda (autoclave digestion);    -   4. filtration and washing of amorphous precipitated silica after        precipitation of amorphous silica; and    -   5. final clarifying filtration after dissolution of amorphous        precipitated silica in sodium silicate solution (boosting the        modulus).

According to various embodiments, the present teachings provide a highlyefficient and improved process for producing precipitated silicic acidproducts that exhibit lower wet cake moisture and/or higher percentsolids, than products previously available, and which can be highlysuitable for boosting the modulus of sodium silicate solutions. Anadditional property of the precipitated silica of the present teachingsis the wet cake low moisture content. In some embodiments the moisturecontent can be at most 55% by weight, or equal to or less than 50% byweight.

According to various embodiments of the present teachings, silica sandscrubs generated as waste during a titanium dioxide pigment productionusing the chloride process, which are inexpensive and possess desirableproperties, can be utilized as a cost-effective precursor of silicondioxide for use in a hydrothermal production of sodium silicatesolutions. In some embodiments, the silica sand scrubs can becharacterized by the following physical and chemical properties, aresummarized in Table 2 below:

TABLE 2 Typical Physical and Chemical Properties of SSS Before and AfterPurification After washing and sieving Before Chemical Wash AnalysisSieve Analysis Silica sand Silica sand ASTM % % Analyses Scrubs ScrubsMesh passed cumulative Al, ppm 1300 1000  +25 37.12 37.12 Ca, ppm 17001700 −25 +35 37.90 75.02 Fe, ppm 350 250 −35 +80 24.30 99.32 Cr, ppm 103  −80 +100 0.37 99.69 Mn, ppm 5 5 −100 +140 0.24 99.93 Ni, ppm <2 <2−140 +170 0.05 99.98 Pb, ppm <2 <2 −170 +200 0.01 99.99 K, ppm 400 —−200 0.01 100.00 Mg, ppm 400 — — — — Ti, ppm 58000 550 — — —

The silica sand scrubs can be further characterized by x-ray diffraction(XRD) and energy-dispersive x-ray spectroscopy (EDX) analyses, theresults of which show that scrubs can be built up purely of quartzmodification of silica only. In some embodiments, TiO₂ impurities can bepresent as crystals on the surface of the quartz particles and notevenly distributed as whole particles.

According to various embodiments of the present teachings, disposingsilica sand scrubs that have been released and collected as waste in atitanium dioxide by the chloride manufacturing process, can beaccomplished simultaneously by a process of converting silica sandscrubs into value-added products, such as, for example, sodium silicatesolutions, silica gels, and/or precipitated silicas, as describedherein. Sodium silicate solutions, also known as soda water glass, canbe used for industrial purposes such as for the production of fillers(precipitated silicas), as adhesives, as binders in paints, as foundryaids, as catalyst supports, as a component of detergents, and as aconstituent of refractory materials. Precipitated silicas can be used,for example, as white reinforcing fillers in the rubber and plasticsindustries, and as additives in paints, coatings, varnishes, lacquers,papers, cosmetics, pharmaceuticals, feed and pesticide industries.

It will be understood that some general objects of the present teachingsare to provide novel resources of cost-effective silica precursors forthe hydrothermal production of sodium silicate solutions having a highSiO₂:Na₂O molar ratio, and processes for preparing the same. Inaddition, it is an object of the present teachings to provide processesfor producing new silicate and silica compositions and also to providesuch compositions themselves. Also, it is an object of the presentteachings to provide advancements in waste utilization.

DEFINITIONS

As used herein, the term silica sand scrubs (SSS), is defined as silicasand solids which are used to scrub and remove build-up from theinterior of flue pipes downstream from a TiO₂ oxidation section of achloride process reactor.

The expression “modulus” of a sodium silicate solution is defined as themolar ratio of SiO₂ to Na₂O and is obtained by the following expression:

Modulus R=(% SiO₂/% Na₂O)×(1.032)

The term “structure” of a precipitated silica is defined as the abilityof a silica material to hold water in the wet cake after the precipitatehas been filtered.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teachings will be more fully understood with reference tothe appended drawings, which are exemplary only and not intended tolimit the scope of the present teachings. In the drawings,

FIG. 1 is a flow diagram of a 2-step reaction process for producingsodium silicate solution, according to various embodiments of thepresent teachings;

FIG. 2 is a flow diagram of a process for producing high modulus sodiumsilicate and precipitated silica, according to various embodiments ofthe present teachings;

FIG. 3 is a graphical representation of the variation of the SiO₂:Na₂Oratio, with time of reaction, in an autoclave, according to variousembodiments; and

FIG. 4 illustrates the relationship between time, and shows internaltemperature and pressure, of an autoclave run according to variousembodiments.

DETAILED DESCRIPTION

The present teachings relate to a hydrothermal process for producingsodium silicate solutions from silica sand scrubs. In FIG. 1, a generalprocess according to an embodiment of the present teachings is shown.FIG. 2 shows another embodiment of the process, according to the presentteachings.

According to various embodiments, a method for hydrothermally producingsodium silicate solution and precipitated silica can utilize rawmaterials comprising silica sand scrubs generated and collected as wastein a titanium dioxide manufacturing process using the chloride process,and utilizing caustic soda (aqueous NaOH), and an acidifying agent. Thecaustic soda and the acidifying agent can be chosen in a manner that iswell known to one of ordinary skill in the art. The caustic sodacomprise, for example, conventional caustic soda produced byelectrolysis of sodium chloride or hydrogen chloride, and having aconcentration of about 50% by weight.

The acidifying agent can comprise, for example, hydrochloric acid(hydrogen chloride), such as conventional hydrochloric acid produced byelectrolysis of sodium chloride, or a by-product from the chlorinationunit of a TiO₂ pigment manufacturing reactor that uses the chloridemanufacturing process. Using a hydrochloric acid by-product, having aconcentration of from about 20% to 25% by weight, can provide benefitsof lower cost, by-product utilization, and pollution abatement.

According to various embodiments, sodium silicate compositions can beproduced and provided that exhibit a SiO₂:Na₂O molar ratio in the rangeof from about 2.0 to about 2.8, in an intermediate stage, and a molarratio in the range of from about 3.0 to about 3.8 in a modulated stage.

Preparation of Silica Sand Scrubs as Feedstock for Sodium Silicate

According to various embodiments of the present teachings, the abovedescribed silica sand scrubs can be used as a feedstock in thepreparation of sodium silicate. The silica sand scrubs can containappreciable TiO₂ on the surface of the particles. In some embodiments,the method can comprise preparing scrubs as a feedstock for an autoclavedigestion.

In some embodiments, particulates of silica sand scrubs in a TiO₂aqueous slurry can contain up to 9.7% surface-adsorbed TiO₂ uponsubsequent recovery. This TiO₂ presence on the surface can affect thequality of the resulting sodium silicate solution, particularly withrespect to clarity, which problems can be overcome by shear washing andsieving operations.

According to various embodiments, a process for preparing feed stockfrom silica sand scrubs can comprise four specific main stages: shearwashing; filtration; drying; and sieving.

An objective of the shear washing step can be to exfoliate and removeTiO₂ pigment particles adhering to the surface of the silica sand scrubsparticles so that the cleaned sand can be used for a subsequent reactionstep to manufacture sodium silicate solution. It has been found thatfiltration and washing conditions have a strong influence on the overallwashing results.

Silica sand can be treated to remove residual TiO₂ particles, andconventional methods can be used to apply shear to the sand particles.Shearing action can be applied by appropriate methods, for example, bymechanical means, by pressure alteration, or by ultrasonic treatment,all known to one of ordinary skill in the art. For example, the aqueousslurry can be sheared by mechanical methods in which the adhering TiO₂can be sheared by use of mechanical means, such as by stirrers. Inanother example, shear can be achieved by ultrasonic techniques in whichcavitations or resonant vibrations cause portions of the particles tovibrate or to be excited at different phases and thus subjected toshear. These methods of shearing are merely representative of usefulmethods, and any suitable method known to one of ordinary skill in theart can be used for shearing.

In some embodiments, exfoliation can be sufficiently high such thatabout 80% to about 99% by weight, or at least about 85% by weight, atleast about 90% by weight, at least about 99% by weight of TiO₂ contentcan be delaminated from the surface to form a slurry comprising TiO₂particles substantially dispersed in an aqueous carrier. The dispersedparticles can be subsequently removed from the slurry by filtration.

As described above, in some embodiments the shear washing of the sandcan be followed by filtration to remove exfoliated and dispersed TiO₂.The washings can be done at increased solids content, for example, witha ratio of water to sand amount in the range of from about 1:0.92 toabout 1:0.85.

According to some embodiments, the pH of the slurry can be adjusted toachieve a level in the range of from about 6.0 to about 9.0. The mixturecan be stirred under high shear conditions using, for example, a highspeed stirrer. Under high shear conditions at high solids content,residual TiO₂ deposits on the surface can be broken up by high shearstress. Washing can also involve introducing baffles in the washingvessel. Under such washing conditions, substantially all TiO₂ residuescan be removed by increasing the mixing speed of the mixer. In anexemplary process, the agitation time can be from about two to aboutfive minutes with a stirrer speed in a range of from 600 rpm to 1000rpm, or in the range of from about 600 rpm to about 800 rpm.

To improve the transfer conditions from the washing vessel to thefiltration unit in order to achieve a constant flow of a homogeneoussuspension, a vessel with a large bottom outlet nozzle can be used, forexample, an outlet nozzle having a diameter than is at least 10% or atleast 25% the diameter of the vessel.

According to various embodiments, results can be achieved based on thefact that the filtration area and the sand cake height can bear animportant relationship with respect to the amount of TiO₂ content in thecake. In some embodiments, filtration performance can be achieved with afiltration area in the range of from 100 cm² to 1000 cm², and acorresponding filter cake thickness in the range of from 10 mm to 70 mm,which can result in a silica sand comprising a TiO₂ content in the rangeof from 1.0% to 1.5% by weight. In some embodiments, a filtration areain the range of from 200 cm² to 800 cm² can be used with a correspondingfilter cake height in the range of from 20 mm to 50 mm, which can resultin a silica sand comprising a TiO₂ content in the range of from about0.13% to about 0.16% by weight. In some embodiments, filtrationperformance can be achieved, with a filtration area of 700 cm² and acorresponding filter cake height of 31 mm, which can result in a silicasand comprising a TiO₂ content in the range of from about 0.05% to about0.10% by weight.

According to various embodiments, the specific sand mass on thefiltration area can be in the range of from about 25 kg/m² to about 100kg/m² filtration area, for example, in the range of from about 25 kg/m²to about 75 kg/m² filtration area, or about 50 kg/m² filtration area.

It has been found that when washed and dried sand is sieved on a 1 mmsieve, practically all contaminating impurities are removed. Bypracticing this sieving step only approximately 2% of the total weightof the washed sand was lost in the tailing.

The removed fraction, comprising particles that did not pass through a 1mm sieve, exhibited the typical analysis shown below in Table 3.

TABLE 3 Typical Analysis of washed sand fraction >1 mm Ti Al Ca Fe Cr MnNi Pb 4.8% 0.68% 0.35% 0.51% 40 ppm 35 45 5 (TiO₂: 8%) ppm ppm ppm

Autoclave Reaction and Following Filtration of Residuals

According to various embodiments, the silica sand scrubs recovered andcollected as wastes that are high in silicon content can be disposed ofby converting them to silicates and silicas. More particularly, theserecovered materials can have desirable properties as a result of theirprevious thermal shock history of going through a thermal treatmentduring their use as scrubbers in the titanium tetrachloride oxidizer ofa chloride process reactor. This treatment can ease a subsequentautoclave reaction and the main impurity can be TiO₂ pigment that can bereadily washed off the sand.

According to various embodiments of the present teachings, a method forpreparing sodium silicate solution can be accomplished by pretreatingsilica sand scrubs of the type described above by reacting them withcaustic soda in a pressurized autoclave. From kinetic constantcalculation, at 200° C., it has been found that the maximum ratio ofSiO₂:Na₂O obtainable, using an infinite time of reaction, is 2.6. FIG. 3shows the variation of the SiO₂:Na₂O ratio with time of reaction.

According to various embodiments, the formation of sodium silicate brothcan be carried out according to the following method. A solid silicasand can be reacted hydrothermally with caustic soda in an autoclavehaving a digestion condition that converts reactants to sodium silicate.The reaction solution does not necessarily contain any silicateinitially. Conventional caustic can be suitable for this reaction.Generally, the washed sand and caustic soda can be mixed instoichiometric quantities. A mixture thus prepared can be pressuredigested in an autoclave at temperatures ranging from 180° C. to 300°C., for a period of from about 4 hours to about 16 hours, for example,at a temperature of from about 200° C. to 250° C., or 225° C., and for atime of from about 4 hours to about 4.5 hours. FIG. 4 depicts aninternal temperature and pressure, against time, graph for an autoclaverun according to various embodiments.

In some embodiments, the core of the experimental set-up can be anINCONEL 600® high pressure autoclave having a diameter (D) of 110 mm, aheight (H) of 350 mm, and a total volume (V_(total)) of 3.3 liters (L).The autoclave can be equipped with a turbine stirrer having 6 blades anda diameter (D) of 55 mm.

In some embodiments, a reactor can be used that is equipped withelectrical heating, temperature/pressure indicators, a temperaturecontroller, a speed controller, and a nitrogen supply. A prepurifiedsilica sand scrub can be first charged into the open reactor vessel.Then, the reactor can be reassembled and a sodium hydroxide solution andwater can be charged into the reactor vessel at ambient temperature. Theaqueous sodium hydroxide solution can comprise from about 15% by weightto about 30% by weight sodium hydroxide, for example, from about 25% byweight to about 30% by weight sodium hydroxide Next, the reactor can bepressure tested with nitrogen and left with a nitrogen pad of between 4bars and 8 bars in order to avoid condensation on the unheated top ofthe autoclave during the run. After the pressure test, a stirrer andelectrical heating can be switched on, thus heating the mixture insidethe vessel under agitation. The reactor temperature can be set between180° C. and 225° C. causing a pressure in the reactor of from about 24bars to about 35 bars (including the initial nitrogen pad). When thetemperature reaches a set value, the reaction can be held at a settemperature and for a set duration before the reactor contents arecooled down and depressurized to atmospheric pressure. Then, theresulting reaction broth can be siphoned out, with nitrogen pressure,through a dip tube and into an unheated vacuum filter unit where it canbe filtered at about 95° C. to separate all the unreacted inert materialfrom the solution.

In some embodiments, INCONEL 600®, titanium, and nickel can be used asexemplary materials for the construction of the autoclave, for example,the autoclave can comprise nickel or a nickel material.

In some embodiments, the silicate broth filtration operation can beperformed by a heated pressure cartridge using appropriate filter cloth.

According to various embodiments of the present teachings, an exemplaryautoclave digestion process can be carried out according to thefollowing reaction conditions:

-   -   1. The speed of the stirrer can be in the range of from 700 rpm        to 1200 rpm, for example, 700 rpm;    -   2. The reaction temperature can be in the range of from 180° C.        to 250° C., for example, 225° C.; and    -   3. The reaction time can be in the range of from 4 hours to 4.5        hours, for example, 4 hours;

In some embodiments, this can accomplish a sodium silicate solution ofthe following results:

1. A yield of 91.2% to 96.4%;

2. A modulus of 2.1 to 2.8;

3. A SiO₂ content of 29.9% to 33.8% by weight; and

4. A Na₂O content of 11.3% to 14.2% by weight.

Precipitation of Amorphous Silica

According to various embodiments, and in a second aspect of the presentteachings, amorphous silica can be prepared from a sodium silicatesolution as a starting material for a precipitation reaction. In someembodiments, by-product hydrochloric acid can be used for theprecipitation reaction. Precipitation of amorphous silica can requireavoiding, during its preparation, any conditions of macroscopic gelformation. There can be a close relation between formation of silica geland of precipitate. In precipitation, silica concentration can be lowerand particles can be brought together into aggregates by forces ofcoagulation. In the absence of a coagulant, silica might not beprecipitated from solution at any pH, because the particles might bemutually repulsive. Coagulation can result from reduction in thedouble-layer repulsion, for example, by decreasing the surface potentialby changing pH, or by increasing the concentration of electrolyte. Thesefacts can be used in a precipitation process for silica. When silicaparticles are present in a hot suspension at a pH of from 9 to 10, theycan be coagulated when the concentration of sodium ion present exceedsabout 0.3N.

In some embodiments, a sodium silicate solution to be used as a startingmaterial in this second aspect of the present teachings can have a ratioof from about 2.1 to 2.8 SiO₂:Na₂O, and exhibit a sodium normality ofabout 0.13C, where C is the silica concentration in grams per 100 mL.

In acidic solution, silicic acid can polymerize to extremely smallparticles that chain together to form a network of gel throughout thewater. At low concentrations, this can collect into weak gelatinousmasses, but otherwise fills the whole volume. In alkaline solution,silica can polymerize to discrete colloidal particles that grow innanometer size range and remain as a stable sol. In some embodiments,precipitated silica particles can be prepared that comprise aggregatesof ultimate particles of colloidal dimensions that have not at any pointexisted as macroscopic gel during the preparation.

According to various embodiments of the present teachings, precipitatedsilica particulates can be prepared in a cake that contains a low amountof water. The properties of the particulates make them readily adaptedfor use as a modulus-boosting silica source material for sodium silicatesolutions. The low amount is compared to a conventional silica filtercake that generally contains a surprisingly high amount of water.

According to various embodiments, a process for the preparation of aprecipitated silica according to the present teachings will now bedescribed. A starting material for such process can comprise a sodiumsilicate solution of low modulus, such as, for example, a sodiumsilicate solution prepared as described above. Silica particulatesproduced in the manner described above can be particularly suitable foruse as starting materials.

In some embodiments of the present teachings, a process is provided forthe preparation of a precipitated silica from a sodium silicate solutionwherein silica particles can be generated by neutralizing a silicatesolution with an acid, and colloidal silica particles can be enabled togrow in weekly alkaline solution. The colloidal silica particles can beflocculated by sodium ions of a resulting sodium salt suitable forboosting the ratio of SiO₂:Na₂O. In some embodiments, the presentteachings provide a process for the preparation of precipitated silicathat advantageously will have better dissolution properties when used ina sodium silicate solution to boost the modulus of the sodium silicatesolution. The precipitated silica of the present teachings can have alow structure. The term “structure” is defined as the ability of asilica material to hold water in the wet cake after the precipitate hasbeen filtered. Accordingly, a structure grading of 65 or greater can beused. Herein, the ranges of >85 to 95, from 75 to 85, and from 65 to 75,are referred to as very high structure, high structure, and mediumstructure, respectively. According to various embodiments, the presentteachings can provide low structure precipitated silica suitable, forexample, for the purposes mentioned herein above.

According to various embodiments, the preparation of precipitated silicacan comprise reacting sodium silicate solution of Step-2 having acomposition in which the weight ratio of SiO₂ to Na₂O is from about 2.1to 2.8, with an acidifying agent, to produce a suspension of aprecipitated silica. The suspension can then be separated. In someembodiments, the precipitation can comprise:

-   -   A. Forming an initial seed-containing silicate base stock        prepared by an initial charge of a silicate solution, the        concentration of silicate in the base stock being controlled by        the pH of solution and without containing any electrolyte such        as the salt of the seeding silicate and the acidifying agent,        for example, without containing (or free of) any sodium chloride        in the case of the reaction of sodium silicate with hydrochloric        acid;    -   B. Simultaneously adding acidifying agent and sodium silicate        solution to the reaction medium under a first reaction medium pH        (pH1) of less than 9, then at a second reaction medium pH (pH2)        of between 2 and 7, at a particular feed rate, while maintaining        a reaction medium precipitation temperature of from 60° C. to        90° C.;    -   C. Digesting and maturing the reaction medium under the same        temperature and pH conditions to allow all the Na₂O added to be        neutralized, and regulating the final pH of the silica to a        desired value for a given application; and    -   D. Separating the silica suspension by filtration followed by        washing.

In some embodiments, the silicate can be the sodium silicate of Step-2described above, having a molar ratio between 2.1 and 2.8.

The acidifying agent can comprise a strong mineral acid such as, forexample, sulphuric acid, nitric acid, or hydrochloric acid, or anorganic acid such as, for example, acetic acid, formic acid orcarboxylic acid, or a combination thereof. In some embodiments theacidifying agent can comprise by-product hydrochloric acid produced froma titanium dioxide manufacturing method using the chloride process. Theacidifying agent can be dilute or concentrated.

According to some embodiments of the present teachings, the reaction canbe performed at a constant temperature. For example, the temperature ofthe reaction medium can be maintained at a temperature between 60° C.and 90° C., between 70° C. and 90° C., or between 80° C. and 85° C., forexample.

According to various embodiments, precipitation can be carried out usingthe following steps:

-   -   E. Pre-filling a certain amount of water into a vessel as a        reaction medium, and heating it up to a desired temperature, for        example, to a temperature of from 60° C. to 90° C., of from        70° C. to 90° C., or of from 80° C. to 85° C.;    -   F. Adding an amount of sodium silicate solution to the        pre-filled water until a pH of 8.5 is reached, thereby forming        an initial silicate seed;    -   G. Simultaneously adding acidifying agent and a sodium silicate        solution with a constant dosage rate for a period of from about        60 minutes to about 70 minutes, maintaining the pH at a level of        from 8 to 10, for example, at 8.5. Adjustments can be made to        correct for pH, for example, slight variations can be made as to        the amount of acidifying agent that is added. The acidifying        agent can be dilute or concentrated; for example, the        concentration can be from about 10% by weight to about 50% by        weight. The acidifying agent can comprise hydrochloric acid, for        example, by-product hydrochloric acid resulting from the        production of TiO₂ using the chloride process, and its        concentration can, for example, be in the range of 10% by weight        to 25% by weight, or in the range of from 20% by weight to 25%        by weight;    -   H. The simultaneous addition can be continued for a period of        from about 10 minutes to about 500 minutes, for example, from        about 60 minutes to about 70 minutes. Addition can be stopped        and the resulting slurry can be allowed to age for a period of        one minutes to about 200 minutes, for example, from about five        minutes to about 60 minutes, or from about 10 minutes to about        20 minutes;    -   I. Additional acidifying agent or acid can be added until the        reaction medium reaches a pH of from 4 to 6.5, for example, a pH        of from 4.5 to 5.5;    -   J. The slurry-containing precipitated silica suspension can be        further digested and the reaction mixture can be matured, for        example, for a period of time of from about one minute to about        100 minutes, from about one minute to about 30 minutes, or from        about three minutes to about five minutes, to attain complete        neutralization and to regulate a final pH of the silica to the        desired value for an intended application;    -   K. The silica slurry produced after the operations described        herein above can be further subjected to filtration, followed by        washing. Filtration can be carried out using any convenient        method known to one of ordinary skill in the art, for example by        means of a belt filter, a rotary vacuum filter, a filter press,        or a pressure filter, with residual water content less than        about 50%. The filtration can be advantageously effected by a        continuous vacuum belt filter.

In some embodiments, the suspension of precipitated silica thusrecovered as filter cake can be used as such and a drying step inbetween precipitation and boosting might not be necessary for theoverall process, in some embodiments of the present teachings.

In some embodiments of the process of preparation, the suspension canexhibit, immediately before filtration, a solids content in a range from5% by weight to about 50% by weight, from 10% by weight to about 30% byweight, or about 30% by weight, to improve washability of the silicacake.

In some embodiments, the filter cake to be dried can have a watercontent of no greater than 55% by weight.

Boosting of Sodium Silicate Solution

Silicas prepared by using embodiments of a process of the presentteachings can be utilized as precursors for the synthesis of highmodulus sodium silicate as described hereinunder, for example inboosting the modulus of sodium silicate solutions from low modulus (inthe range of from 2.1 to 2.8) to a modulus of high value (in the rangeof from 3.0 to 3.8).

According to various embodiments, a process for producing anintermediate sodium silicate solution with a SiO₂:Na₂O molar ratio ofless than about 3.0:1 can be directly combined with a subsequentreaction mixture comprising intermediate sodium silicate solution withadded precipitated silica to produce a boosted sodium silicate solutionhaving a SiO₂:Na₂O molar ratio of from 3.0:1 to 3.8:1 as an end product.

An exemplary process for preparing boosted sodium silicate solutionaccording to various embodiments, will now be further described. To anautoclave reaction product, after depressurizing the autoclave, aquantity of precipitated silica can be introduced. The quantity ofsodium silicate added can correspond to the additional amount of SiO₂required to establish a molar ratio of SiO₂:Na₂O of from 3.0:1 to 3.8:1,in the sodium silicate solution desired as the end product. The reactorcan then be left at a reaction temperature for about another 20 minutesto 60 minutes, with stirring. The soda water glass solution can then befurther processed, for example, in a sedimentation vessel, for thecoarse separation of solids, or, where clarity of the solution has tosatisfy more stringent requirements, in a filter apparatus.

In some embodiments, the pressurized liquid phase of the sodium silicatesolution can be transferred to a second optionally preheated reactionvessel, into which a calculated quantity of precipitated silica has beenintroduced, and the reaction can be completed therein, boosting themodulus of the sodium silicate solution obtained in the autoclavereaction step. On completion of the reaction, water can additionally beadded directly into the reactor or into a separate dissolution vessel.In some embodiments, the dissolution can be carried out in the samepressure vessel at the required conditions of temperature and agitation,or in a separate vessel.

EXAMPLES

The following specific examples are provided to more particularlyillustrate the present teachings and are not to be construed aslimitations thereon.

Washing and Filtration of the Sand—Examples 1-3

These examples illustrate the initial preparation of SSS recovered froma titanium dioxide slurry, the resulting sand containing TiO₂ in anamount of up to about 9.1% by weight.

Deionized (DI) water was taken in a 7 liter volume beaker (diameter(D)=155 mm) with a central bottom outlet that was closed with a plug.The vessel was equipped with 4 baffles and a 6 blade Ruston—Turbine thatwas installed 20 mm above the bottom of the vessel. In order to achievea homogenous suspension in the vessel, an additional 3-bladePitched-Blade Turbine was installed. With the agitator switched on, SSSwere charged into the vessel and agitated for 5 minutes at from 600 rpmto 800 rpm. The filling ratio of the vessel height/diameter (H/D) was inthe order of 0.8 to 1.0. At the end of 5 minutes agitation and while thestirrer was running, the plug was removed so that the SSS suspensiondrained onto the filter immediately under the bottom outlet and wasallowed to drain for about 20 seconds. The cake was washed four timeswith water. The wash water was added to the filter cake. The washed andfiltered sand was dried in an oven at 105° C. and finally sieved througha one millimeter (mm) sieve. The fractions greater than one mmconstituted about 2% of the total weight, and were rejected.

Three samples (1, 2 & 3) were prepared. The washing and filtrationdetails for preparing samples 1, 2, and 3 are set forth in Table 4 andTable 5 below. Table 6 gives the analytical results of the dry sand.

TABLE 4 Experimental Details Example 1 Example 2 Example 3 Vol ofvessel, L 7 7 7 Diameter of vessel, mm 155 155 155 Stirrer Type RushtonRushton Rushton Turbine Turbine + Turbine + 3-blade 3-blade PitchedPitched Blade Turbine Blade Turbine Baffles (4x) Yes yes yes StirrerDiameter, mm 70 70 + 100 70 + 100 Speed, rpm 800 710 600 DI water, g3500 3500 3500 Quartz Sand (wet), g 3850 3850 3850 Agitation Time, min 55 5 Estimated Specific Power 6.6 14.4 8.7 Input (P/V), W/L

TABLE 5 Washing and Filtration Data Example 1 Example 2 Example 3 FilterArea, cm² 700 700 700 Filter Cloth 05-1005 K-115 05-1005 K-115 05-1005K-115 Water Wash 1, g 2750 2750 2750 Water Wash 2, g 2750 2750 2750Water Wash 3, g 2750 2750 2750 Water wash 4, g 3100 3100 3100 CakeHeight, mm 31 31 30 Sand (Moist), g 3395 3386 3349 Residual Moisture, %6 7 6 Sand (Dry), g 3186 3138 3132

TABLE 6 Analytical Results (Dry Sand Sample) Original Sand: UnwashedExample 1 Example 2 Example 3 & sieved Ti, % 0.077 0.058 0.055 5.8 Al, %0.10 0.10 0.10 0.13 Ca, % 0.15 0.15 0.17 0.17 Fe, % 0.05 0.05 0.0350.025 Cr, ppm 15 20 10 3 Mn, ppm 5 5 5 5 Ni, ppm <5 <5 <5 <2 Pb, ppm <5<5 <5 <2

The result shows that the silica sand scrubs can be washed substantiallyfree of TiO₂.

Autoclave Reaction and Filtration of Residuals Examples 4-6

Washed, dried and sieved sand was first charged into the open reactorvessel. Then the reactor was reassembled and the sodium hydroxidesolution and water were charged into the reactor at ambient temperature.Next, the reactor was pressure tested with nitrogen and left with anitrogen pad of between 4 bar-a and 8 bar-a in order to avoidcondensation on the unheated top of the autoclave during theexperimental run. After the pressure test, the stirrer and electricalheating were switched on, thus heating the mixer inside the vessel underagitation. When the temperature reached the set value, the reaction washeld at the set temperature and duration before the reactor contentswere cooled down and depressurized to atmospheric pressure and theresulting reaction broth was siphoned out. The reaction broth wassiphoned with nitrogen pressure through a dip tube and into an unheatedvacuum filter unit where it was filtered at about 95° C., separating allthe unreacted inerts from the solution.

Tables 7 and 8 below provide summaries of autoclave operations,filtration details, and analytical data corresponding to Examples 4-6.

TABLE 7 Summary of Autoclave Reaction Runs Example 4 Example 5 Example 6Reactor Material of Inconel 600 Inconel 600 Titan Construction InternalsCooling Cooling N/A Coil Coil Target Temp., ° C. 225 225 225 Speed ofStirrer, rpm 1200 1200 700 Initial Nitrogen pressure, bar 8.0 6.8 4.0Reaction Time at Target 4 4 4 Temp, Hr Sand 905 g 1050 g 510 g SA 407202SE 408262 MC 409231 100 g MA 409091 110 g MA 409061 Sodium Hydroxide(50.4%), g 1171 1054 723 DI water at start of run, g 1042 1045 717 DIwater added after reaction — — — time, g

TABLE 8 Summary of Filtration Data Example 4 Example 5 Example 6 FilterType vacuum vacuum pressure Unit Area, cm² 200 200 616 Filter Cloth B 46MU 100 B 46 MU 100 PP2450 Temperature of Filter 20 (no jacket) 70 90Jacket, ° C. Filtration Time, min >120 165 8 Filtration Pressure, bar0.5 0.2 4 Filtrate, g 2810 3314 1830 Yield after reaction, % 94.5 91.296.4 Yield after Filtration, % 94.1 90.2 86.5 SiO₂ content, % 29.9 28.233.8 Modulus 2.1 2.5 2.8

The moduli of the sodium silicate solutions were in the range from 2.1to 2.8.

Precipitation of Amorphous Silica Example 7

1.2 L of water were introduced into an indirectly heated 4 Lprecipitation vessel and heated to 85° C. while being stirred. The pHwas initially adjusted to 8.5 while maintaining this temperature byadding a little water glass solution (weight modulus 2.8:1=24.6% SiO₂and 9.1% Na₂O; density 1.405 g/ml). Precipitation was then performed for107 minutes by simultaneously adding 2100 g of sodium silicate at a rateof 19.6 g/min of water glass (composition as stated above) and asufficient quantity of 23% hydrochloric acid to ensure that the pH washeld constant at 8.5. The solution was allowed to age for 20 minutes.This suspension was then acidified to pH 5.5 with 23% hydrochloric acidand aged for 5 minutes. The silica obtained was separated from thesuspension using a Larox pressure filter at 4 bar-g pressure, the filtercake was washed two times with water wherein each time 500 grams ofwater was used, until the washed filtrate contained less than 10 g/Lchloride content. The weight of filter cake obtained was 1136 g. Thesilica content of the suspension was 26.8% by weight, calculated basedon the weight of the wet silica after filtration to the total weight ofthe slurry. The wet silica filter cake contained 39.2% water when driedat 110° C. for 2 hours.

The purity of the precipitated silica was 99.5% SiO₂ when analyzed byx-ray fluorescence (XRF).

Example 8

670 g of water was introduced into an indirectly heated 2.5 Lprecipitation vessel and heated to 80° C. while being stirred. The pHwas initially adjusted to 8.5 while maintaining this temperature byadding a little water glass solution (weight modulus 2.8:1=21.3% SiO₂and 7.9% Na₂O). Precipitation was then performed for 71 minutes bysimultaneously adding 1390 g sodium silicate at a rate of 19.6 g/min ofwater glass (composition as stated above) and a sufficient quantity of20% hydrochloric acid to ensure that the pH was held constant at 8.5.The solution was allowed to age for 20 minutes. This suspension was thenacidified to pH 5.5 with 20% hydrochloric acid, and aged for fiveminutes. The silica obtained was separated from the suspension using apressure filter at one bar-g pressure, and the filter cake was washed 3times with water, each time using 284 g water. The weight of the filtercake obtained was 597 g. The silica content of the suspension was 23% byweight, calculated based on the weight of the wet silica afterfiltration to the total weight of the slurry. The wet silica filter cakecontained 49.2% water when dried at 110° C. for 2 hours. The purity ofthe precipitated silica was 98.7% SiO₂ when analyzed by XRF.

Boosting of the Sodium Silicate Solution Examples 9-12

In this group of examples, the precipitated silica sand in Example 7 andExample 8 was used to boost the SiO₂:Na₂O molar ratio of the aqueoussodium silicate solutions produced in Example 4, Example 5, and Example6. The experiments were conducted with different batch sizes varyingfrom 959 g to 2169 g of water glass. The amorphous silica used for theboosting experiments was not dried after the precedingprecipitation/filtration steps. Therefore the moisture content of theamorphous silica was between 45% by weight and 56% by weight.

The dissolution temperature was varied between about 75° C. and 92° C.The dissolution time at the lowest temperature of about 75° C. was about10 minutes, after reaching the temperature, and was almost the same withall batches. In some embodiments of the present teachings, thedissolution temperature can be from about 80° C. to about 92° C., forexample, about 80° C. or about 85° C.

Final clarifying filtration was performed, directly after boosting, in apressure filter thermostated at about 90° C. The filtration time wasabout 10 min. with all samples. A heated cartridge filter was used forfinal clarifying filtration.

The SiO₂:Na₂O ratio obtained varied from 3.0 to 3.8. The preparationconditions, filtration data, and analytical details of the final sodiumsilicate solutions are summarized below in Table 9, Table 10, and Table11.

TABLE 9 Experimental Details Example 9 Example 10 Example 11 Example 12Vol of Glass Vessel, L 4 4 4 4 Vessel Diameter, mm 145 145 145 145Stirrer Type Rushton Rushton Rushton Rushton Turbine Turbine TurbineTurbine Baffles No No No No Position of Stirrer eccentrical eccentricaleccentrical eccentrical and tilted and tilted and tilted and tiltedDiameter of Stirrer, mm 58 58 58 58 Speed, rpm 350-400 500 400 400Distance Stirrer to Bottom, mm 25 25 25 25 DI Water, g 686.7 319.3 126243.8 Water Glass from Autoclave Reaction, g 1350 2161 1277 959Nomenclature Water Glass RB407211 RB408311 RC409211 RC410041 AmorphousSilica (wet), g 230.5 382.6 112.2 220 Nomenclature Amorphous SilicaFA409071 FA409131 FA409291 FC410071 Sodium Hydroxide Solution from N/AN/A N/A N/A Cristal, g Dissolution Temp., ° C. 92 92 75 92

TABLE 10 Filtration Data Example Example Example 9 Example 10 11 12Filter Type pressure pressure pressure pressure Unit Area, cm² 125 125125 125 Filter Cloth G11U004 G11U004 G11U004 G11U004 Temperature of 9090 89-95 95 Filter Jacket, ° C. Filtration Time, min 8 13 65 abortedafter 150 Filtration Pressure, bar 1 1.5 2-3 3 Amount of Filtrate, g2227 2822 1469 1016

TABLE 11 Analytical Data Example 9 Example 10 Example 11 Example 12 SiO₂content, % 27.2 28.7 28.9 27.6 Modulus, R 3.0 3.8 3.3 3.4 Ti, ppm 195100 240 1400 Al, ppm 265 240 370 370 Ca, ppm 23 30 10 20 Fe, ppm 98 100140 60 Cr, ppm 40 40 8 5 Mn, ppm <5 <5 <5 <5 Ni, ppm <5 <5 <5 <5 Pb, ppm<5 <5 5 <5

The final modulus of the sodium silicate solutions obtained was in therange from 3.0 to 3.8.

Example 13 Preparation of Sodium Silicate R=3.52

A sodium silicate with R=2.56 was reacted with precipitated silica at80° C., obtaining a product with a raised value of R=3.52. Thecomponents of the reaction mixture were:

SS, R = 2.56 (SiO₂ = 30.27%; Na₂O = 12.18%) 245.7 g Precipitated Silica(SiO₂ = 95%)  29.2 g Water  75.1 g Total 350.0 g

The filtered solution had the following composition (by weight):

R 3.52 °Be 40.55 SiO2 29.17% Na2O 8.55%

Example 14 Preparation of Sodium Silicate R=3.51

A sodium silicate solution with R=2.80 was reacted with precipitatedsilica at 80° C. obtaining a product with a raised value of R=3.51.

The components of the reaction were:

SS, R = 2.8 (SiO₂ = 24.6%; Na₂O = 9.1%) 1000 g Precipitated Silica (SiO₂= 95%)  150 g Water  80 g Total 1230 g

The filtered solution had the following composition:

SS, R 3.51 SiO2 24.48% Na2O 7.19%

While the present teachings have been described in terms of exemplaryembodiments, it is to be understood that the changes and modificationscan be made that fall within the scope of the present teachings.

1. A sodium silicate solution produced by a method comprising the stepsof: (i) manufacturing titanium dioxide, wherein the manufacturingcomprises generating a waste product, the waste product comprising aprecursor solid silica source; (ii) collecting the waste product; (iii)exfoliating and removing titanium dioxide particles adhering to thesurface of the precursor solid silica source of the collected wasteproduct, to form cleaned silica; (iv) producing an intermediate sodiumsilicate solution by a hydrothermal fusion reaction of the cleanedsilica with a caustic soda in an autoclave; (v) forming an amorphoussilica source by transposing the SiO₂ content of the intermediate sodiumsilicate of step (iv) by a separate precipitation process; and (vi)dissolving the amorphous silica provided in step (v) in the intermediatesodium silicate solution of step (iv) and raising the SiO₂:Na₂O molarratio to result in a high modulus sodium silicate solution, wherein thesodium silicate solution is characterized by the following properties: aphysical form of a clear and colorless liquid; a density of from 1.35kg/L to 1.45 kg/L; a SiO₂,% of 24.5% by weight, minimum; a Na₂O,% of9.0% by weight maximum; and a SiO₂:Na₂O molar ratio of from 3.0 to 3.8.2. The sodium silicate solution of claim 1, wherein the SiO₂:Na₂O molarratio is in a range of from about 3.0:1 to about 3.8:1.
 3. The sodiumsilicate solution of claim 1, wherein the precursor solid silica sourceof the waste product collected in step (ii) comprises silica sand scrubsfrom a titanium dioxide manufacturing process using chloride.
 4. Thesodium silicate solution of claim 1, wherein the hydrothermal fusionreaction of step (iv) comprises reacting the precursor solid silicasource of the waste product collected in step (ii) with a caustic sodain a closed autoclave pressure reactor, at a temperature range of fromabout 180° C. to about 300° C., and under a saturated steam.
 5. Thesodium silicate solution of claim 4, wherein the caustic soda comprisesan aqueous sodium hydroxide solution, and the amounts of the precursorsolid silica source and the aqueous sodium hydroxide solution are suchthat the molar ratio of SiO₂ in the precursor solid silica source todissolved Na₂O in the aqueous sodium hydroxide solution is in a range offrom about 2.0 to about 2.8, with respect to
 1. 6. The sodium silicatesolution of claim 4, wherein the aqueous sodium hydroxide solutioncomprises dissolved Na₂O in an amount stoichiometrically equivalent to amolar ratio of Na₂O: SiO₂ in a range of from about 3.0 to about 3.8,with respect to 1, and the aqueous sodium hydroxide solution comprisesfrom about 10% to about 50% by weight sodium hydroxide.
 7. The sodiumsilicate solution of claim 1, wherein the intermediate sodium silicatesolution is produced in step (iv) by a hydrothermal fusion reaction ofan aqueous sodium hydroxide solution with the cleaned silica formed instep (iii) in an autoclave pressure vessel at a temperature in the rangeof from about 180° C. to about 300° C., and under a saturated steam. 8.The sodium silicate solution of claim 7, wherein the aqueous sodiumhydroxide solution comprises from about 15% to about 30% by weightsodium hydroxide.
 9. A high modulus sodium silicate solution produced bya method comprising the steps of: (i) manufacturing titanium dioxide,wherein the manufacturing comprises generating a waste product, thewaste product comprising a precursor solid silica source; (ii)collecting the waste product; (iii) exfoliating and removing titaniumdioxide particles adhering to the surface of the precursor solid silicasource of the collected waste product, to form cleaned silica; (iv)producing an intermediate sodium silicate solution by a hydrothermalfusion reaction of the cleaned silica with a caustic soda in anautoclave; (v) forming an amorphous silica source by transposing theSiO₂ content of the intermediate sodium silicate of step (iv) by aseparate precipitation process; and (vi) dissolving the amorphous silicaprovided in step (v) in the intermediate sodium silicate solution ofstep (iv) and raising the SiO₂:Na₂O molar ratio to result in a highmodulus sodium silicate solution.
 10. Precipitated amorphous silica,comprising the following chemical characteristics: Analyses Wt % SiO₂53.3705 Na₂O 0.2646 H₂O 45.000 TiO₂ 0.0642 Al₂O₃ 0.0831 CaO 0.0054 Fe₂O₃0.0236 Cr₂O₃ 0.0080 MnO₂ 0.0005 NiO 0.0007 PbO 0.0006 HCl 0.0000 NaCl1.1787 Total 100.00